Keel mounted turret

ABSTRACT

A vessel ( 1 ) includes a relatively small sized turret ( 2 ) located near keel level. The upper part ( 3 ) of the turret wall is located below a level halfway between deck level ( 12 ) and keel level ( 7 ). Via a relatively small diameter product shaft ( 9 ), the product pipe ( 18 ) which connects to a riser ( 17 ) extends up to deck level ( 12 ). The lower part ( 6 ) of the turret wall extends along the height of the chain table ( 8 ), which is connected thereto via an axial and a radial bearing ( 23, 24 ). In this way the chain table ( 8 ) can adequately take up the bending moments exerted by the mooring chains ( 13 ) or cables. After use of the vessel ( 1 ) as a floating production storage and offloading vessel, the lower part ( 6 ) of the turret wall can be easily removed and the vessel ( 1 ) can be converted back to a transport tanker. Preferably the lower part ( 6 ) of the turret wall is releasably connected to the upper part ( 3 ) by means of releasable bolts ( 27 ) through the upper slide bearings ( 23 ) of the chain table ( 8 ).

BACKGROUND OF THE INVENTION

The invention relates to a vessel comprising a turret extendingvertically within the hull of the vessel, the turret having acylindrical turret wall with an upper part and a lower part and anannular attachment element which is rotatingly connected to the lowerpart of the turret wall for the connection of one or more anchor linesfor mooring the vessel to the sea bed, the lower part of the turret wallbeing located at or near keel level, wherein the upper part of theturret wall is located at a position which is below the level halfwaybetween deck level and keel level.

From U.S. Pat. No. 5,266,061 a vessel is known comprising a largediameter turret wall that extends from deck level vertically downwardsinto a moon pool located near keel level. Within the fixed turret wall,a lower turret section is rotatingly connected. The lower turret sectioncomprises a chain table which is connected to the sea-bed in ageostationary manner. Risers, which extend from a sub-sea structure suchas a well head, enter the turret through the chain table and extend intoa central shaft extending up to deck level, the central shaft beingrotatable together with the chain table within the turret wall. Duringweathervaning of the vessel, the outer turret wall will rotate togetherwith the vessel around the geostationary chain table, the lower part ofthe turret and the central shaft accommodating the product pipes.

Providing a large diameter turret wall inside the vessel, the diameterof which may amount up to between 15 and 20 meters, is an extensiveoperation which may cause a structural weakening of the vessel and whichrequires separate reinforcements. Futhermore, once a vessel is providedwith such a large turret construction for conversion of a tanker to afloating production, storage and offloading vessel (FPSO), it can noteasily be rebuilt and brought back into its original state as atransport tanker.

From U.S. Pat. No. 5,237,948 a vessel is known comprising a mooringsystem. Herein the chain table is fixed to a rigid shaft connected tothe hull of the vessel. The chain table is rotatably supported on aflange on the shaft. This construction has as a disadvantage that theknown chain table will be susceptible to bending moments which can bedirectly transferred to the bearings.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a relatively simpleweathervaning construction on a vessel for connecting to a sub-sea oilstructure during hydrocarbon exploration activities, which can berelatively easily converted back to its original state. It is a furtherobject of the present invention to provide a vessel with a turretconstruction which is of low cost, and which can be easily accessedduring installation and on conversion. It is another object of thepresent invention to provide a weathervaning mooring construction whichhas a large resistence against bending moments.

Thereto a vessel according to the present invention is characterised inthat a shaft extends from at least the upper part of the turret walltowards deck level for accommodating a product pipe which is connectedto a sub sea structure, the diameter of the shaft being substantiallysmaller than the diameter of the turret wall, wherein the lower part ofthe turret wall extends along substantially the whole height of theannular attachment element, the attachment element being supported by anupper and a lower bearing, one of the bearings being an axial bearing,the other bearing being a radial bearing.

By providing a relatively short turret wall within the lower half of thevessel, a relatively simple structure is formed which can duringhydrocarbon exploration effectively function as a turret structure, andwhich can be easily installed and removed from the vessel. As thetopmost part of the weathervaning construction is formed by the shaftwith a relatively small diameter for accommodating the product pipes,relatively little structural weakening of the vessel occurs, and littleuseful storage space inside the vessel is occupied. As the annularattachment element, such as a chain table according to the presentinvention is along its full length supported by the turret wall by meansof an axial and a radial bearing, a structure is obtained which canadequately take up the bending moments exerted by the mooring chains orcables, without the risk of deformation of the bearings and subsequentfailure of the chain table to rotate. After use of the vessel forhydrocarbon exploration, for instance when the hydrocarbon well has beendepleted, the part of the turret wall close to keel level can beremoved, for instance by cutting through the turret wall, but preferablyby means of a releasable coupling. Preferably the lower part of theturret wall extends below keel level such that the attachment element,such as a chain table, can be easily accessed for installation orremoval. It is however possible to accommodate the chain tablecompletely within a recess in the hull of the vessel, such that it isprojecting below the outer circumference of the vessel.

Preferably the height of the turret wall is relatively small and theupper part is located relatively close to keel level, such as at adistance of between 0.1 and 0.5 times the distance between keel leveland deck level. The diameter of the shaft substantially corresponds withthe diameter of at least one product pipe for allowing rotationalmovement of the product pipe within the shaft. Near deck level, theproduct pipe is connected to a swivel which connects a horizontalproduct pipe in a rotatable manner to the product pipe extending in theshaft of the vessel. It is however also possible to mount the swivelnear or at the turret position by lowering the swivel through the shaftfrom deck level. In that case the diameter of the shaft is made largeenough to accommodate the swivel.

In a preferred embodiment of the vessel according to the presentinvention, the turret wall comprises an upper and a lower bearing, eachbearing having a radially extending flange with an axial bearing surfaceand an axially extending part forming a radial bearing surface, theaxially extending part of the upper bearing being fixedly connected tothe lower part of the turret wall, the radially extending flange of theupper bearing being fixedly connected to upper part of the turret wall,the flange and the axially extending part of the upper bearing beingmutually connected in a releasable manner.

By decoupling the upper bearing part, for instance by means of bolts,the lower part of the turret according to the invention including theattachment element can be easily be moved. With the releasable bearingsaccording to the invention it is also possible to exchange the lowerpart of the turret for instance after damage to the chain table, or foruse with a differently configured chain table that is adapted tospecific operating conditions at different locations.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the vessel according to the present invention will beexplained in detail with reference to the accompanying drawings. In thedrawings:

FIG. 1 shows a partly cross-sectional view of a vessel comprising aturret according to the present invention and

FIG. 2 shows a detail of the bearings of the upper and lower parts ofthe turret of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a vessel 1 which near its bow is provided with a turret 2.The turret 2 comprises an upper part 3 comprising a cylindrical turretwall 4 which extends in a vertical direction within the hull 5 of thevessel. The lower part 6 of the turret 2 is located below keel level 7and comprises an attachment element or chain table 6. A vertical shaft 9extends through the upper and lower parts 4,6 of the turret verticallyupwards through the vessel towards deck level 12. In a preferredembodiment, the shaft 9 consists of an upper and a lower shaft part thatare connected to each other by means of a coupling near the turret 2.This coupling has not been shown in the drawing.

An anchor chain 13 is connected to the chain table 8 via a chain stopper14. The end of the anchor chain 13 is connected to a cable runningthrough the chain pull tube 15 and being guided via a sheave 16 to achain installation winch (which is not shown in the drawing). The otherend of the anchor chain 13 is connected to the sea-bed via an anchor oran anchoring pile. A riser 17 is connected to the chain table 8 and isconnected to a product pipe 18. The product pipe 18 extends within theshaft 9, the diameter of which is only slightly larger than the diameterof the product pipe. The product pipe 18 is positioned within the shaft9 by means of pipe guides 19,19′ located along the length of the shaft9. At deck level 12, the product pipe 18 is connected to a swivel 20. Itis however also possible that the swivel 20 is mounted below deck level12, for instance at or near the position of the turret 2. Via a rotatingring of the swivel the geostationary product pipe 18, around which theshaft 9 can rotate, is connected to horizontal product pipes 21 on thedeck of the vessel 1. The diameter of the turret wall 4 is about 4 m.The diameter of the shaft 9 is about 60 cm. The distance between decklevel and keel level is about 25 m.

The upper part 3 of the turret 2 extends one third of the height betweenkeel level 7 and deck level 12 into the vessel 1. The lower part 6 ofthe turret 2 is releasably connected to the upper part 3 by means ofcoupling means 22. The coupling means 22 are part of the upper bearings23 of a pair of axial and radial bearings 23,24 between the turret wall4 and the rotating chain table 8. Although the coupling means 22 in FIG.1 are shown to project below keel level 7, it is preferred that they arelocated above keel level, such that after detaching the lower part 6 ofthe turret wall 4, no parts project from below the vessel 1. The detailsof the bearings 23,24 are shown in FIG. 2.

As can be seen in FIG. 2, the upper part 3 of the turret wall isprovided with a bearing 23 comprising a flange 25 and a radial supportring 26. The flange 25 of the upper bearing 23 is welded to the upperpart 3 of the turret wall 4. The radial support ring 26 is welded to thelower part 6 of the turret wall. The flange 25 and the ring 26 arereleasably connected by means of bolts 27. The lower bearing 24comprises a flange 28 and a radial support ring 29 which are connectedin a non-detachable way. The outer axial and radial surfaces of thebearings 24,25 are provided with a low friction coating or low frictionpads to form axial and radial slide bearings for the bearing rings 30,31of the chain table 8.

What is claimed is:
 1. Vessel comprising: a turret extending verticallywithin a hull of the vessel, the turret having a fixed cylindricalturret wall with an upper part and a lower part and an annularattachment element which is rotatingly connected to the lower part ofthe turret wall for the connection of one or more anchor lines formooring the vessel to a sea bed, the lower part of the turret wall beinglocated at or near keel level, the upper part of the turret wall beinglocated at a position which is below the level halfway between decklevel and keel level; and a shaft extending from at least the upper partof the turret wall towards deck level for accommodating a product pipewhich is connected to a sub sea structure, a diameter of the shaft beingsubstantially smaller than a diameter of the turret wall, the lower partof the turret wall extending along substantially the whole height of theannular attachment element, an upper part of the attachment elementbeing supported by an upper bearing, adjacent to an upper surface of theattachment element a lower part of the attachment element beingsupported by a lower bearing, adjacent to a lower surface of theattachment element one of the upper and lower bearings being an axialbearing, the other of the upper and lower bearings being a radialbearing.
 2. Vessel according to claim 1, wherein the lower part of theturret wall extends below keel level.
 3. Vessel according to claim 2,wherein the diameter of the shaft is smaller than 0.5 times the diameterof the turret wall.
 4. Vessel according to claim 1, wherein the diameterof the shaft is smaller than 0.5 times the diameter of the turret wall.5. Vessel according to claim 1, wherein a distance of the upper part ofthe turret wall from keel level is between 0.1 and 0.5 times thedistance between keel level and deck level.
 6. Vessel according to claim1, wherein the diameter of the shaft substantially corresponds with thediameter of the product pipe for allowing a relative relational movementof the product pipe with respect to the shaft.
 7. Vessel according toclaim 1, wherein the lower part of the turret wall is releasablyconnected to the upper part of the turret wall.
 8. Vessel according toclaim 7, wherein the turret wall comprises coupling means located nearkeel level connecting the upper and the lower parts of the turret. 9.Vessel according to claim 8, wherein each of the upper and lowerbearings has a radially extending flange with an axial bearing surfaceand an axially extending part forming a radial bearing surface, theaxially extending part of the upper bearing being fixedly connected tothe lower part of the turret wall, the radially extending flange of theupper bearing being fixedly connected to upper part of the turret wall,the flange and the axially extending part of the upper bearing beingmutually connected in a releasable manner.
 10. Vessel according to claim1, wherein the upper bearing is directly adjacent to an uppermostsurface of the attachment element and the lower bearing is directlyadjacent to a lowermost surface of the attachment element.
 11. Vesselaccording to claim 1, wherein the upper and lower bearings are spacedapart in a direction generally parallel to a longitudinal axis of theturret.
 12. Vessel comprising: a turret extending vertically within ahull of the vessel, the turret having a cylindrical turret wall with anupper part and a lower part and an annular attachment element which isrotatingly connected to the lower part of the turret wall for theconnection of one or more anchor lines for mooring the vessel to a seabed, the lower part of the turret wall being located at or near keellevel, the upper part of the turret wall being located at a positionwhich is below the level halfway between deck level and keel level; anda shaft extending from a least the upper part of the turret wall towardsdeck level for accommodating a product pipe which is connected to a subsea structure, a diameter of the shaft being substantially smaller thanthe diameter of the turret wall, the lower part of the turret wallextending along substantially the whole height of the annular attachmentelement, the attachment being supported by an upper and lower bearing,one of the bearings being an axial bearing, the other bearing being aradial bearing, the lower part of the turret wall being releasablyconnected to the upper part of the turret wall, wherein each of theupper and lower bearings has a radially extending flange with an axialbearing surface and an axially extending part forming a radial bearingsurface, the axially extending part of the upper bearing being fixedlyconnected to the lower part of the turret wall, the radially extendingflange of the upper bearing being fixedly connected to upper part of theturret wall, the flange and the axially extending part of the upperbearing being mutually connected in a releasable manner.